Traits that are common among psychopathic serial killers—a grandiose sense of self-worth, persuasiveness, superficial charm, ruthlessness, lack of remorse and the manipulation of others—are also shared by politicians and world leaders. Individuals, in other words, running not from the police. But for office. Such a profile allows those who present with these traits to do what they like when they like, completely unfazed by the social, moral or legal consequences of their actions.

If you are born under the right star, for example, and have power over the human mind as the moon over the sea, you might order the genocide of 100,000 Kurds and shuffle to the gallows with such arcane recalcitrance as to elicit, from even your harshest detractors, perverse, unspoken deference.

“Do not be afraid, doctor,” said Saddam Hussein on the scaffold, moments before his execution. “This is for men.”

If you are violent and cunning, like the real-life “Hannibal Lecter” Robert Maudsley, you might take a fellow inmate hostage, smash his skull in and sample his brains with a spoon as nonchalantly as if you were downing a soft-boiled egg. (Maudsley, by the way, has been cooped up in solitary confinement for the past 30 years, in a bulletproof cage in the basement of Wakefield Prison in England.)

Or if you are a brilliant neurosurgeon, ruthlessly cool and focused under pressure, you might, like the man I'll call Dr. Geraghty, try your luck on a completely different playing field: at the remote outposts of 21st-century medicine, where risk blows in on 100-mile-per-hour winds and the oxygen of deliberation is thin. “I have no compassion for those whom I operate on,” he told me. “That is a luxury I simply cannot afford. In the theater I am reborn: as a cold, heartless machine, totally at one with scalpel, drill and saw. When you're cutting loose and cheating death high above the snowline of the brain, feelings aren't fit for purpose. Emotion is entropy—and seriously bad for business. I've hunted it down to extinction over the years.”

Geraghty is one of the U.K.'s top neurosurgeons—and although, on one level, his words send a chill down the spine, on another they make perfect sense. Deep in the ghettoes of some of the brain's most dangerous neighborhoods, the psychopath is glimpsed as a lone and merciless predator, a solitary species of transient, deadly allure. No sooner is the word out than images of serial killers, rapists and mad, reclusive bombers come stalking down the sidewalks of our minds.

But what if I were to paint you a different picture? What if I were to tell you that the arsonist who burns your house down might also, in a parallel universe, be the hero most likely to brave the flaming timbers of a crumbling, blazing building to seek out, and drag out, your loved ones? Or that the kid with a knife in the shadows at the back of the movie theater might well, in years to come, be wielding a rather different kind of knife at the back of a rather different kind of theater?

Claims like these are admittedly hard to believe. But they're true. Psychopaths are fearless, confident, charismatic, ruthless and focused. Yet, contrary to popular belief, they are not necessarily violent. Far from its being an open-and-shut case—you're either a psychopath or you're not—there are, instead, inner and outer zones of the disorder: a bit like the fare zones on a subway map. There is a spectrum of psychopathy along which each of us has our place, with only a small minority of A-listers resident in the “inner city.”